(1) Filed of the Invention
The present invention lies in the area of electric machines. It relates to a reversible electric machine with multiple air gaps and a 3D magnetic flux. Such an electric machine may be monophase or polyphase.
(2) Description of Related Art
An electric machine that converts electrical energy into mechanical energy—for example, for the propulsion of a vehicle—is called a “motor”. An electric machine that converts mechanical energy into electrical energy—for example, for the generation of an electric current—is called a “generator”. Among generators, a distinction can be drawn between alternators, which supply an alternating electric current, and dynamos, which supply a direct electric current.
An electric motor may be adapted in order to be fed by a direct electric current or by a monophase or polyphase alternating electric current, such as a three-phase electric current. Similarly, an alternator may be adapted to generate a monophase or polyphase alternating electric current.
However, a polyphase alternating electric current must be balanced in order to allow smooth and fluid operation of the electric machine. Such a balanced polyphase alternating electric current, thus forming a balanced electrical system, includes at least three phases and is characterized, in particular, by the fact that the sum of the complex voltages (or currents) of each phase is null, while the amplitude of the voltage (or current) of each phase is not simultaneously null. Moreover, an identical phase shift is present between each phase of this current.
An electric machine is said to be reversible when it can be simultaneously used as a motor and a generator. Any electric machine can be reversible, with the distinction between the motor and generator functions being made only with regard to the purpose and the use of this electric machine. The term “motor-generator” is also used if both functions are available on the electric machine.
The motors currently in use may be rotary (that is, producing angular displacement and/or torque) or linear (producing linear displacement and/or a force).
On the other hand, generators are essentially rotary.
A rotating electric machine is an electromechanical device that includes at least one stator that is fixed and at least one rotor that rotates with respect to the stator, and which can be located inside or outside the stator. The rotation of this rotor is generated by the interaction between two magnetic fields that are attached to this stator and the rotor respectively, thus creating a magnetic torque on the rotor. Thus, the phrases “stator magnetic field” and “rotor magnetic field” are used respectively.
Because the remainder of this description will be limited to rotating electric machines, for the sake of simplicity the term “electric machine” will be used to designate a rotating electric machine. Similarly, the term “electric motor” will designate a rotating electric motor, and the term “generator” will designate a rotating electric generator.
The various electric machine technologies are distinguished essentially by the way in which the stator and rotor magnetic fields are generated.
For example, in a direct-current electric motor, the stator includes magnetic elements, which may be permanent magnets or non-permanent magnets, more commonly known as electromagnets, and which typically consist of one or more windings of electric conductors supplied with a direct electric current. The term “coil” will be used in this document to designate a set of one or more windings of electric conductors. In both cases, each magnet includes two poles (a north pole and a south pole), and a fixed stator magnetic field is thus created. Conversely, the rotor includes non-permanent magnets consisting of a coil that creates a rotational magnetic field when a direct electric current passes through it. When this rotor rotates, a rotating collector makes it possible to reverse the direction of this direct electric current passing through the rotor coil at least once per rotation, thus reversing the poles of the non-permanent magnets of this rotor and thereby modifying the direction of the rotor magnetic field.
Thus, a shift between the stator and rotor magnetic fields causes a magnetic torque on the rotor, with, for example, a north pole of the stator repelling a north pole of the rotor and attracting a south pole of the rotor. Consequently, a rotation of the rotor with respect to the stator is generated.
A principal disadvantage of such a direct-current electric motor resides in the electrical contacts that are necessary between the rotor coil and the rotating collector. These contacts, which are obtained for example by means of brushes, can create electric arcs that in particular cause wear, and parasitic currents that consequently require frequent maintenance schedules for the electric machine. Furthermore, this type of electric motor is not suitable for high rotation speeds, and consumes energy due to friction, thereby reducing its performance. Finally, it can be complex to implement.
These disadvantages have been eliminated thanks to brushless-motor technology, also known as “brushless motors”.
The rotor of such an electric machine includes one or more permanent magnets, while the stator includes a coil including non-permanent magnets. Such a machine may also include means for determining the position of the rotor (for example, through the use of a sensor), as well as an electronic control system that ensures the switching of the electric current. An alternating electric current then circulates within the stator coil. Thus, this electronic control system makes it possible to ensure the orientation and the direction of the stator magnetic field with respect to the rotor magnetic field, and consequently the rotation of the rotor with respect to the stator, with the rotating stator field engaging the rotor field.
Furthermore, within the stator coil, one or more windings may be grouped in order to form different stator phases, with each phase having an identical phase shift with respect to the other phases. In motor mode, each phase is fed by one phase of a polyphase alternating electric current and, respectively, generates a stator magnetic field, with each stator magnetic field that is associated with a phase likewise being shifted with respect to the other stator magnetic fields that are associated with the other phases. The stator magnetic fields, when they are derived from a single polyphase electric current forming a balanced electric system, add up to form a single stator magnetic field, known as a “stator resultant”, which rotates at a synchronous frequency. This stator resultant then causes the rotation of the rotor field, and consequently creates a rotation of the rotor with respect to the stator.
Similarly, in generator mode, the rotation of the rotor causes the rotation of the rotor field and the creation of a rotating stator resultant, which is decomposed into one magnetic field for each phase of the stator, thus generating the appearance of a polyphase alternating electric current.
Among electric machines that use alternating electric current, a distinction can be drawn between synchronous and asynchronous electric machines.
Synchronous electric machines, which include brushless motors, have a rotor that includes one or more permanent magnets and a stator that includes a coil provided with multiple windings that may form one or several phases. In fact, when one or more of the alternating electric currents of a balanced polyphase electric system pass through them, the windings of the stator coil create one or more rotating stator magnetic fields, whose stator resultant engages the rotor magnetic field at the synchronous frequency of the machine, thereby causing the rotation of the rotor.
Conversely, a rotation of the rotor, generated by an external mechanical force, creates a rotation of the rotor magnetic field, which causes the creation of one or more rotating stator magnetic fields forming the stator resultant, and, consequently, the appearance and the circulation of one or more alternating electric currents in the stator coil.
The permanent rotor magnets may be replaced by a coil that is fed by a direct electric current, forming non-permanent magnets and thus creating a rotor magnetic field. The direct electric current may be delivered by an electric current generator, such as a battery or a capacitor.
The rotational frequency of the rotor of a synchronous electric motor is proportional to the frequency of the alternating electric current applied to the stator. Similarly, the frequency of the alternating electric current generated in a synchronous generator is proportional to the rotational frequency of the rotor. The synchronous machine is often used as a generator, for example, as an alternator in electric power stations.
Asynchronous electric machines have a rotor that includes a coil whose windings are short-circuited and a stator that includes a coil, forming non-permanent magnets. In fact, when an alternating electric current passes through this stator coil, it creates one or more rotating stator magnetic fields, whose stator resultant causes the appearance of a rotor electric current in the rotor coil, thereby generating a magnetic torque on this rotor and, consequently, the rotation of this rotor with respect to the stator.
Conversely, a rotation of the rotor generated by an external mechanical force will cause the appearance and the circulation of an alternating electric current in the stator coil. In order for this to occur, the electric machine must be connected to a network that includes, for example, at least one inverter and one battery, in order to supply it with the reactive energy that is necessary for its operation in generator mode.
Although the rotational frequency of the stator magnetic field is proportional to the frequency of the alternating electric current passing through the stator coil, the rotational frequency of the rotor of an asynchronous electric motor is not necessarily proportional to this frequency of the alternating electric current, because a slip rate may appear between the rotor and the stator magnetic field. Similarly, the frequency of the alternating electric current generated in an asynchronous generator is not necessarily proportional to the rotational frequency of the rotor.
For a long time, asynchronous machines were used only as electric motors, for example, in the transportation field, to drive ships and trains, as well as in the industrial area for machine tools. Thanks to the use of power electronics, such electric machines are also used today as generators—for example, in wind turbines.
Furthermore, the use of such reversible electric machines on board vehicles, such as automobiles or rotary-wing aircraft, is being developed for the implementation of a hybrid motor installation using two types of energy (both thermal and electrical) to drive the vehicle. However, the use of these electric machines today is limited by certain constraints, such as the power-to-weight ratio of the machines and of the electric-energy storage means.
Regardless of the type of reversible electric machine, a magnetic flux circulates between the rotor and the stator through the various permanent or non-permanent magnets of this rotor and of this stator, and this flux is channeled by the magnetic poles of these magnets. In fact, this magnetic flux circulates from a north pole to a south pole across the air gap located between each pole of the stator and of the rotor, as well as between a south pole and a north pole in the vicinity of the stator and of the rotor.
Furthermore, the rotor magnets—whether they are permanent or non-permanent, may be oriented in two ways, thus leading to at least three types of electric machines.
On the one hand, the magnets may be oriented perpendicular to the axis of rotation of the electric machine, that is, with the two poles of each magnet oriented perpendicular to this axis of rotation. These magnets are said to be radially oriented, or are simply referred to as radial magnets. A radial magnetic flux is thus created in the vicinity of these magnets, i.e., perpendicular to this axis of rotation. Thus, the air gap in the vicinity of these magnets is arranged to lie parallel to this axis of rotation.
On the other hand, the magnets may be oriented parallel to the axis of rotation of the electric machine—that is, with the two poles of each magnet oriented parallel to this axis of rotation. These magnets are said to be axially oriented, or are simply referred to as axial magnets. An axial magnetic flux is thus created in the vicinity of these magnets, i.e., parallel to this axis of rotation. Thus, the air gap in the vicinity of these magnets is located perpendicular to this axis of rotation.
These various orientations of the magnets make it possible to orient the magnetic flux that circulates inside the electric machine, which, in a first type of electric machine (for example, a disc-rotor electric machine) is axial; or, in a second type of electric machine (for example, a cylindrical-rotor electric machine), is radial. For a third type of electric machine, both radial and axial magnets may be used in the same electric machine, such that a magnetic flux is created that is simultaneously radial and axial. This type of magnetic flux is referred to as a “multiple air-gap magnetic flux”. Conversely, regardless of the orientation of the magnets, a single magnetic flux circulates throughout the electric machine.
Contemporary electric machines use various configurations and orientations of the magnetic flux, that is, a radial or axial magnetic flux, in order better to meet the customer's needs, in terms of both performance and dimensions. For example, machines with permanent magnets and a strongly coupled axial flux are shorter axially and larger radially, whereas machines with a radial flux are small radially and long axially.
Furthermore, the power-to-weight ratio of these electric machines—namely, the ratio of their power to their mass—and their manufacturing cost will vary depending on the magnetic-flux configurations that are used, without necessarily being optimal.
Electric machines with prongs and permanent magnets are the most desirable type today, thanks to a high performance/cost ratio in comparison with other machine technologies, particularly because of the use of permanent magnets; the use of a soft, pressed and baked magnetic alloy (i.e., a soft magnetic compound) instead of the assembly of laminated sheets; and the use of axial coils, as well as because they involve a limited number of parts. The use of a soft, pressed and baked magnetic alloy, which notably possesses isotropic characteristics in all three directions, enables the manufacture of complex three-dimensional shapes. However, the power-to-weight ratio is not optimal, because these electric machines use only one orientation of the magnetic flux created from the magnetic field generated by the stator operating in motor mode. In fact, this magnetic flux circulates essentially radially or axially with respect to the axis of the machine. Thus, using this technology, in order to have high torque machines it would be necessary to increase either the radius (for radial machines) or the length (for axial machines) of the air gap, which would result in large and heavy machines. Thus, this technology does not use all of the possibilities for the orientation of the magnetic flux, and does not allow the creation of high torque electric machines that are also compact and light.
The prior art also includes document EP 0613229, which describes a direct-current brushless motor that includes a stator and a rotor. The rotor consists of a circular rotor yoke provided with multiple magnetic poles whose north poles and south poles are arranged in alternation.
The stator includes two assembled circular stator yokes, within which a coil is located. Each stator yoke includes bent tabs, with one bent tab of each stator yoke located alternately opposite a magnetic pole of the rotor yoke, thus forming an air gap. Furthermore, when an alternating electric current passes through the coil, the coil magnetizes the bent tabs of each stator yoke, alternately forming a north pole and a south pole. Thus, these different magnetic poles, which are present on the stator and the rotor, make it possible to generate a rotational motion of the rotor. This document describes various ways of ensuring accurate and stable angular positions between the two stator yokes.
Meanwhile, FR 2828027 describes a machine with a homopolar structure that includes a stator and a rotor. The stator includes one or more circular yokes, each of which consists of two identical, angularly indexed crowns, inside of which a coil is located. Each crown includes bent tabs that alternately form a north pole and a south pole when an electric current passes through the coil. When the stator includes multiple yokes, these yokes are separated by a spacer made of a non-magnetic material. Furthermore, in a polyphase motor, each coil may be connected to a different phase.
Meanwhile, EP 1770846 describes an electric machine with a radial flux that includes a stator and a rotor. The rotor includes permanent magnets, while the stator includes one or more circular yokes, each of which consists of two identical, angularly indexed crowns, inside of which a coil is located. Each crown is made of a magnetic powder that is compacted along the direction of the axis of rotation of the crown, and includes prongs that alternately form a north pole and a south pole when an electric current passes through the coil. Three yokes that are phase shifted by 120° are assembled and insulated by an insulating resin, such that a stator for a three-phase electric machine is formed.
Moreover, WO 2004/107541 describes a transverse-flux electric machine that includes a stator and a rotor. The stator includes a coil and two circular yokes located at each end of this coil. Each yoke includes C shaped tabs that are bent on the circumferential face of the coil, along with a multitude of conductive parts located on this circumferential face and distributed uniformly between the C shaped tabs, thus forming several rows along the axial direction of the electric machine. The rotor includes a multitude of magnets that are likewise arranged in several rows along the axial direction of the electric machine, thereby facing each C shaped tab and each conductive part of the stator, forming an air gap with them. When an alternating electric current passes through the coil, the C shaped tabs and the conductive parts are magnetized, and a magnetic flux then circulates radially between the rotor magnets, on the one hand, and the C shaped tabs and the conductive parts of the stator, on the other hand, thereby generating a rotational movement of the rotor.
The prior art also includes CN 101212150, which describes a dual-air-gap electric machine that includes a stator and a rotor. The rotor includes two groups of magnets, with one group being positioned radially and the other group being positioned axially facing the axis of rotation of the electric machine. Similarly, the stator includes a coil and two groups of magnetic poles, with one group being positioned facing radial magnets of the rotor, thereby creating a radial magnetic flux, and with the other group being positioned facing axial magnets of the rotor, thereby creating an axial magnetic flux. In fact, a dual air gap that is both radial and axial is created between the rotor and the stator. The magnetic poles of the stator and of the rotor consist of circumferentially alternating north and south poles. Conversely, an adjacent radial pole and an adjacent axial pole are identical. Thus, the magnetic flux that circulates in the electric machine can divide, in order to pass radially and axially through the dual air gap and circulate between the poles of the stator and of the rotor, then recombine in the vicinity of the stator and of the rotor.
Lastly, FR 2961037 and FR 2959621 relate to a homopolar electric machine that includes a rotor and a stator that has one or more phases. The rotor includes multiple permanent magnets, and each phase of the stator consists of a coil and a yoke provided with alternating magnetic poles, with each yoke being formed by two crowns. More specifically, FR 2961037 describes the shape of the teeth carried by each crown, with each crown forming each magnetic pole of the stator, thereby enabling the optimization of the circulation of the magnetic flux. In fact, such a tooth shape allows the magnetic flux to circulate maximally in the magnetic circuit, which consists, among other things, of the stator yoke, thereby limiting both its circulation in air and magnetic leakage. Meanwhile, document FR 2959621 describes how to perform the angular indexing of the yokes constituting each phase of this homopolar electric machine, by using an intermediate disk located between each yoke. Each intermediate disk includes index fingers that cooperate with holes located in each yoke, in order to ensure the angular offset corresponding to each phase.